1. Field of the Invention
The subject invention relates to controlling the operation of various types of gas discharge lamps, and in particular, an improvement in the operational performance of electronic ballasts within a high frequency range of a gas discharge lamp.
2. Description of the Related Art
High intensity discharge (HID) gas discharge lamps as known in the art suffer from acoustic resonances when such lamps are operated at high frequencies, i.e., between a few kHz and hundreds of kHz, depending on the type of lamp. However, the acoustic resonances significantly weaken in such gas discharge lamps in which the acoustic resonances do not have a negative affect on the performance of these gas discharge lamps when the lamps are operated at very high frequencies, i.e., above the highest acoustic resonance (e.g., 150 kHz for a 400 W metal halide lamp). However, a consequence of operating the gas discharge lamp in the VHF range is the generation of electro-magnetic interference. Additionally, when a gas discharge lamp is operated at VJF lamp currents, the electrode temperature modulation (i.e., the difference in anode and cathode temperatures) vanishes. This results in a different electrode operating condition, which could cause changes in the arc attachment on the electrode. Arc instabilities related with arc-electrode attachment have been found when 400 W metal halide lamps are operated at high frequencies, even up to as high as 500 kHz.
Back-arcing of a gas discharge lamp involves an arc attachment of the arc on the back of the electrode coil of the lamp, as opposed to an ideal arc attachment of the arc at the tip of the electrode. This can affect thermal balance of the end of the arc tube, which, in turn, can affect the vapor pressures. Consequently, the color properties of the lamp are affected.
There are a number of known methods for operating HID lamps stably at high frequencies. A first method is to operate at a current frequency that is below the frequency of the lowest acoustic resonance. This method is limited to very low power lamps because acoustic resonance frequencies scale as one over an inner dimension of the lamp envelope. For higher wattage (larger) lamps, the lowest acoustic resonance frequencies are below 40 kHz power frequency (20 kHz current frequency) and the circuit can produce audible noise. A second method is to find a xe2x80x9cresonance free windowxe2x80x9d that lies between the acoustic resonance frequencies. This method depends critically on the dimensions of the lamp. Small variations in manufacturing tolerances or changes in lamp parameters over the life of the lamp can make this xe2x80x9cwindowxe2x80x9d disappear. A variation on this method is to frequency sweep through a range of weak resonances. Again, the frequency range is very dependent on lamp dimensions. A third method for operating an HID lamp stably, is to increase the frequency sufficiently such that the acoustic resonances are damped. In this case, it is hard to guarantee that very weak resonances will not occur. The frequencies of these weak resonances vary unpredictably from lamp to lamp and can even vary from one operating period to another. Frequency sweeping at VHF has not proven totally successful in eliminating these instabilities.
It is an object of the invention to be able to drive an HID lamp at very high frequencies while eliminating arc instabilities. This object is achieved in a method of driving a high intensity discharge (HID) lamp, comprising the steps generating a very high frequency driving signal for said HID lamp; generating a low frequency modulating signal; amplitude modulating said driving signal with said modulating signal at a level of 10% to 30%; and applying said amplitude modulated driving signal to said HID lamp.
Applicants have found that when the modulating signal has a frequency of substantially 100 Hz, and the driving signal has a frequency in the range of 100 kHz to 500 kHz, stabilization of the arc of the HID lamp is attainable.
Since the properties of each lamp have a direct bearing on the stability, in a preferred embodiment of the invention, the method of driving a high intensity discharge (HID) lamp, comprises the steps generating a very high frequency driving signal for said HID lamp; generating a low frequency modulating signal; amplitude modulating said driving signal with said modulating signal at a predetermined low initial modulation level; measuring a lamp voltage across said HID lamp; determining a standard deviation of said lamp voltage; comparing said standard deviation with a predetermined minimum level; if said standard deviation is above said predetermined minimum level, incrementally increasing said modulation level and repeating said amplitude modulating step, said measuring step, said determining step and said comparing step; and if said standard deviation is below said predetermined minimum level, maintaining said amplitude modulation at said determined level. This method may be modified by first trying the amplitude modulation when the driving frequency is at an initial value. Then, if the standard deviation does not drop below the predetermined minimum level when the amount of amplitude modulation reaches a predetermined amount, the driving frequency may be incrementally increased (or decreased) and the procedure repeated until the appropriate combination of driving frequency and amount of amplitude modulation is achieved.